Many automotive engines are cooled with liquid coolant. The coolant absorbs heat while circulating within the engine and then transfers that heat to ambient air while circulating through a radiator. During operation in the most demanding operating conditions, an engine driven fan may be used to increase the flow of ambient air through the radiator. In less demanding conditions, it is desirable not to operate the fan to reduce the load on the engine. To achieve this intermittent fan operation, the engine crankshaft may drive the fan via either an actively controlled or thermostatically controlled fan clutch.
A fan clutch is illustrated in FIG. 1. Input shaft 10 is driven by the engine crankshaft either directly or via some power transfer mechanism such as an accessory drive belt. Output shaft 12 drives the fan. Input plate 14 is fixed to input shaft 10 while output plate 16 is fixed to output shaft 12 via a clutch cover 18. Ribs on input plate 14 are interspersed with ribs on output plate 16 such that the ribs are close to one another but do not touch. To engage the clutch, a working fluid is released from reservoir 20. As the fluid flows through the narrow gap between the ribs, viscous shear in the fluid exerts torque on the input plate and output plate. This narrow gap is called the working zone. The magnitude of the torque depends upon the relative speed between the plates and on the quantity of fluid in the working zone. When the fluid reaches the perimeter of the working zone, it is moving circumferentially. Some of the fluid enters return channel 22 in the clutch cover. If the output shaft is moving slower than the input shaft, then the fluid slows as it enters the return channel, causing an increase in pressure. When the speed difference between the input and output plates is sufficient, the increased pressure forces the fluid through return channel 22, against centrifugal force, back to reservoir 20. Thus, in the engaged state, fluid circulates continuously from the reservoir, through the working zone, through the return channel, and back to the reservoir. The output shaft speed stabilizes at a speed less than the input shaft speed.
To disengage the clutch, valve 24 is moved into a position in which it blocks the flow of fluid out of the reservoir 20. Once the fluid that was in the working zone exits the working zone, all torque transfer stops. Once the torque capacity is reduced, drag causes the fan to slow down. As the fan slows down, all of the fluid is returned to reservoir 20 through return channel 22. The position of valve 24 may be controlled via an actuator 26. For example, actuator 26 may be a stationary electro-magnetic actuator that pulls valve 24 into the engaged position shown in FIG. 1 by exerting a magnetic force. A return spring 28 pushes the valve into the disengaged position when the magnetic force is removed.